If a mask blank has defects, the mask pattern formed on a photomask cannot be correctly transferred to a semiconductor substrate. It is therefore important to inspect the mask blank before the photomask is prepared.
A method of inspecting a mask blank for defects is known, in which a dark-field optical system acquires dark-field images. If the mask blank has defects, intense scattered light will emanate from the defects. In any dark-field image, the defects are observed as bright points. Hence, in order to detect a defect signal, a threshold level is set, and any signal having a level higher than the threshold level is regarded as a defect signal.
In the course of the inspection, however, noise signals may be detected, erroneously as defect signals. For example, if the threshold level is too low, weak defect signals can be detected, but a frequency of (the number of) erroneously detecting noise signals increases. On the other hand, if the threshold level is too high, fewer noise signals are detected, but defect signals having intensity lower than the threshold level cannot be detected.
Thus, it has hitherto been difficult to detect noise signals accurately in the process of inspecting the mask substrate (e.g., mask blank) for defects. It has inevitably hard to inspect, with high reliability, the mask substrate for defects.